1. Field of the Invention
The invention concerns a method of compensating for decreases or drops in the amplitude of signals read from magnetic record carriers, in particular to drops in servo signals for track following control of magnetic heads, as well as an arrangement for implementing this method.
2. Description of Prior Art
In so-called sector servo systems for track following on magnetic disk files, sectors or bursts of servo information are interspersed around the disk between recorded portions containing data. There are a number of different servo patterns employed in such files, but they all involve detecting the bursts of servo information and using this information to generate a position error signal (PES) which is employed to maintain the magnetic head properly centered over the desired data track.
Whenever there are defects in a magnetic layer, drops in the amplitude of signals read from the magnetic record carriers occur at the very point at which the signal to be read is recorded. This is true whether the defects in the magnetic layer existed from the time of manufacture, or whether they are attributable to scratches that occurred during operation, such as by contact of the layer with the magnetic head. It is obvious that errors in the read signal lead to read errors with serious consequences.
If the read signals are servo signals which are used for the track following control of magnetic heads, then erroneous read signals may lead to faulty position signals and thus to a faulty track following of the magnetic head. U.S. Pat. No. 3,185,972, for example, shows a track following control of a magnetic head by means of servo signals in which the servo signals are sampled at servo marks recorded in servo sectors of the recording tracks. The servo sectors are positioned at identical angular spacings between the data sectors of the magnetic disk. Sampling of the data and servo sectors is effected with the aid of a magnetic head which is adjusted to the track address of a recording track by means of the access unit. For adjusting the track address and for its observance, i.e., the head follows the track to which it has been originally adjusted, the servo system is controlled by switching means synchronized with the disk rotation. As a result, the servo signals read by the magnetic head when the servo sectors are sampled are fed at predetermined intervals to the servo control where they are evaluated and used for control purposes.
U.S. Pat. No. 3,691,543 shows the derivation of the magnetization pattern of a servo track from two adjacent track areas with positive magnetization or flux changes at points arranged in parallel. Between these positive flux changes each track area has a negative flux change which is staggered relative to the negative flux changes of the adjacent tracks. The servo signals for track centering the magnetic head are derived from the negative flux changes that are spaced from each other. The clock signals are derived from the positive flux changes of the servo tracks.
An article in the IBM Technical Disclosure Bulletin, Vol. 13, No. 11, April 1971, page 3505, shows compensating for errors occurring in the servo signals by increasing the length of the defective servo signal marks, thus compensating for their impaired energy content and restoring them to their previous state. In practice this necessitates that after recording on a magnetic record carrier has been completed, the servo marks are accurately read and checked so that they may be appropriately modified and rewritten if defects are detected. This involves great expense on the one hand and makes it impossible on the other to eliminate those defects that are connected only during the operation of the magnetic record carrier, for example, by its surface becoming scratched.
An article of the IBM Technical Disclosure Bulletin, Vol. 22, No. 8A, January 1980, pp 3127-3130, describes a method in conjunction with the accurate recording of servo signals, whereby surface defects occurring after fabrication are detected as the servo signals are recorded and are eliminated by appropriately modifying the servo signal recordings. In accordance with this method, the servo recording of a track is based on a comparison of the servo recordings of the few previously recorded servo signals of adjacent tracks. This applies to the track spacings as well as the signal strength. This method is suitable for eliminating only those servo signal errors that have occurred during recording of the servo signals. Errors subsequently encountered cannot be compensated for.
For compensating for defective areas and for preventing signals being written on to defective areas of magnetic record carriers, the IBM Technical Disclosure Bulletin, Vol. 22, No. 5, October 1979, page 2091, shows including in the data recordings information as to which areas of the record carrier cannot be recorded on. This approach also necessitates that defective areas, before being used as data carriers, are detected and recorded. Defective areas which result only during operation may still lead to errors.